The present invention relates generally to production of thermoplastic tubing and formable plastic products and, more particularly, relates to a method and apparatus for producing small and large diameter thermoplastic tubing and composite tubing structures.
One known method for manufacturing corrugated pipes is to extrude a tube of thermoplastic from a head and then conform that tube to the interior of a corrugated mold tunnel formed by mold blocks. Mold blocks typically come in pairs and mate together to form a portion of the mold tunnel, and the thermoplastic is either blown into shape (by pressure created within the plastic tunnel) or vacuumed into shape (where vacuum draws the air from around the mold tunnel). Typically, the mold blocks operate in a clam shell-type fashion such that each of the mold block pairs pivot on an axis into mating engagement with one another. Examples of clam shell type corrugators are shown in U.S. Pat. Nos. 5,645,871; 5,494,430; 4,439,130 and others. The clam shell-type shape of mold blocks are susceptible to xe2x80x9csawtoothxe2x80x9d imperfections which can occur throughout the mold tunnel. Sawtoothing occurs when successive mold blocks are tilted slightly rather than perfectly aligned with the axis of the mold tunnel. The hinged mold carrier, when closed, has a tendency to lean back in the opposite direction of travel during the molding process. This xe2x80x9csawtoothingxe2x80x9d creates molding lines as well as a potential loss of vacuum.
The clam hell type mold carriers typically require the use of multiple custom-machined components and have a high manufacturing cost. The hinge pin is subject to external forces which results in wear on the hinge pin. A worn hinge pin may not allow the mold to pivot closed properly which could prevent the tubing firm forming properly. Furthermore, wear and misalignment of the pivoting pairs may result in the hinged mold carrier bending or breaking some of the custom-machined components.
At least one prior art device, U.S. Pat. No. 5,510,071 teaches a pair of mold blocks that are operated differently from the clam shell variety. Instead of using a hinged pivot, as is customary in the art, this device allows for transverse linear recipcal travel for closing and opening of the mold block pair. The apparatus taught by the ""071 Patent utilizes reciprocal linear travel for the carriage supporting the mold blocks. There is no teaching in the ""071 Patent to utilize linearly opening and closing of mold block pairs in a continuous loop corrugator.
Typically mold blocks must be maintained within an acceptable range of temperatures during the molding process. However, during the extrusion press, hot plastic is extruded into the mold blocks which tends to heat the blocks. To maintain acceptable temperatures the blocks must be cooled. Cooling of mold blocks maybe a problem. Inadequate cooling of mold blocks may lead to excessive temperatures for the molding process and result in degradation of the quality of molded tubes. Too much cooling may result in crackling of the exterior surface of molded tubes thereby giving the impression of an alligator-like skin on the pipe surface. Traditionally, mold blocks have been water cooled with internal cooling passages within each blocks. Failure of water lines and water leakage is a problem for many corrugator designs. If significant water leaks into the mold cavity during molding, a defective product may result, requiring the corrugator to be shut down, the leak located and the mold block or other leaking component repaired. Accordingly improved cooling techniques are desired.
In order to keep mold blocks in contact with the product to produce good forming characteristics, prior art corrugators utilize chain driven mold blocks where mold blocks are located substantially around the entire perimeter of a continuous path. At least one improvement is illustrated by U.S. Pat. Nos. 5,494,430 and 5,531,583 where a mold train drive and a shuttle drive are utilized to minimize the number of mold blocks. Nevertheless, this system relies on gearing and at least two separate chain drives: one to shuttle molds from the end of the mold tunnel to the beginning of the mold tunnel and one to drive molds through the mold tunnel.
The mechanical drive system utilized by corrugator designs in the prior art typically have mechanical drives comprised of motors, gear boxes, gear reductions, transmissions, sprockets, chains, idler sprockets, power take off shafts and other mechanical drive systems. These drive methods and systems may require extensive maintenance. Furthermore, these drive methods are subject to wear as many components are in direct contact with one another. There is also the potential problem of backlash in the driving mechanisms, which may be accentuated in the propulsion of the molds.
The corrugators such as taught by U.S. Pat. Nos. 5,494,430; 5,645,871; and others are known as vertical style corrugators. A forming mold tunnel is in line with the extrusion head die. The non-forming (return) of the mold chain is typically overhead or below the product center line. As shown in FIG. 2 of U.S. Pat. No. 5,494,430, the molds which are not in use in the mold process are typically open and cannot be used in the manufacture of product until they return to the forming side of the corrugator. This results in a machine having a greater height in the vertical plane and does not utilize the non-forming molds in production. U.S. Pat. No. 5,257,924 teaches at least one method for using multiple molding sections in a corrugator.
Maintaining a vacuum is another concern in many corrugator and mold designs. U.S. Pat. Nos. 4,718,844 and 5,059,109 teach at least two different vacuum configurations for mold blocks. Thermoexpansion of forming molds can cause vacuum loss through the vacuum interface during production. The vacuum interface is typically a rigidly fixed device that interfaces with the molds. If the interface is not complete, vacuum leakage will occur. Although it appears that improvements have been made to the mold blocks for maintaining vacuum, the interface with the vacuum does not appear to have been thoroughly perfected.
With the clam shell style mold carriers, the mold carriers typically operate on an oval track. The radius on the curved portions of the track typically is relatively large. This causes a need for the extrusion die to be of relatively long length. Longer dies have been found to create problems in the manufacture of double walled tubing such as high head pressures and/or uneven distribution of the extruding plastic. Furthermore, long die lengths reduce the potential selection of materials to be utilized which could otherwise be utilized with shorter die lengths.
A need exists to utilize materials such as polyvinylchloride (PVC). Polyvinylchloride (PVC) requires a shorter die length than is available in many prior art corrugators.
When constructing some prior art corrugators, a mold chain length has been fixed. If a customer has a special product, or wants to increase or decrease the length of the machine, the customer could typically expect extensive modifications to be required and performed upon that particular corrugator. Today, there is a high demand for greater and greater speed which often results in longer and longer forming tunnels. For instance, when a 20-foot mold section was once found adequate, a consumer now may desire a 40-foot mold section. This modification may be extensive and require a lengthy down time for the machine.
The present invention recognizes and addresses the foregoing disadvantages, and others, of prior art construction and methods. Accordingly, it is an object of the present invention to provide an improved machine, such as a corrugator, for making plastic tubing or other formed plastic product. A trackway defining a continuous path and a plurality of carriages for carrying mold blocks about the continuous path are utilized. Each of the carriages transports at least one pair of mold blocks which are linearly displacable from one another. In a closed configuration, a first and second pair element of a pair of mold blocks engages with one another along a molding section of the continuous path to form an axial portion of a mold tunnel in the molding section. Using the linear closing mechanism, the molds may be operated on a continuous path which has a quick, i.e., smaller, entrance radius. A small entrance radius in a continuous path allows for extrusion dies to be shortened. With a die of shorter length, the traditional problem of die length is reduced. Accordingly, rapidly solidifying materials such as polyvinylchloride (PVC) may be utilized to make corrugated tubing.
The mold blocks open and close preferably using a non-hinged carrier. This carrier is comprised of a main plate with two linear slot bearings which are mounted in a manner to allow the mold to open evenly away from the product. Accordingly, the carrier need not hinge away from itself. The linear non-hinged mold carrier, is preferably composed largely of xe2x80x9coff-the-shelfxe2x80x9d components which may provide for quick field repairs without the need to remachine mold locating surfaces. Furthermore, the radius and angle of curvature of the continuous track may assist in correct positioning of opposing mold blocks when forming the mold tunnel without having mold surfaces wear against one another.
The mold blocks may be balanced by use of sprockets and chain. Movement of one block may drive the opposing block in a similar fashion (i.e., inward or outward). The balanced design is believed to assist in prolonging the life of components and may assist in faster production rates due to lower power consumption.
In an alternative embodiment, linear motors operate in conjunction with programmable logic controls to engage a select number of carriages at the beginning of the mold tunnel to continuously drive the carriages along the mold tunnel. The linear motor working with programmable logic controls is also able to shuttle carriages which are not in the mold tunnel along portions of the continuous path at a higher rate of speed than the speed of the mold blocks along other sections of the continuous path. Linear motors may be used independently of mechanical drive mechanisms such as gear boxes, chains, transmissions, and the like. The linear motor may accelerate a carriage which leaves the mold tunnel, shuttle the out-of-service mold block through a second section of the continuous path, decelerate the carriage prior to the beginning of the mold tunnel and maintaining a predetermined speed, speeds, and/or positions through the mold tunnel. The more preferable method of moving the carriages is to use a drive rod and sprocket to drive at least one at a time with the driven carriage pushing the others about the continuous loop.
The mold block may be utilized in conjunction with a vacuum machine for molding tubes. In a preferred embodiment, the tube product is corrugated. The mold block is preferably adapted to moving about an endless path and cooperates with other blocks to define a molding tunnel with a vacuum source along a portion of the path. In an alternative embodiment, at least two molding tunnels may be formed at two different portions of the continuous path.
The mold blocks may include a plurality of sub-blocks secured end to end. Each sub-block has a face arcuate about a longitudinal axis and corrugated with at least one complete wave length of circumferential grooves and lands. A vacuum channel within the mold block provides access from a vacuum source to the interior portion of the mold. The vacuum passage is connected to a vacuum source at a vacuum interface as the mold block passes along the mold tunnel. The vacuum interface in the molding section or sections preferably utilizes a cushion. This cushion may be pneumatic or mechanical such as springs and allows for the interface to adjust as the forming molds thermally expand without binding or leakage.
The corrugator utilizes at least one molding section and may include a shuttle section. In a transition section, a one or more tracks direct the mold carrier and the mold blocks to a closed position where they are maintained through a molding section. A second transition section contains one or more tracks which may assist in guiding the mold blocks to an open position to allow the extruded product to be removed. Preferably, the track(s) and rollers prevent xe2x80x9csawtoothingxe2x80x9d by properly aligning a pair of mold blocks in a closed configuration during the molding section.
Preferably the machine is constructed in a horizontal fashion instead of a vertical planer fashion such that the molds move in a horizontal fashion. The vertical profile is lowered by not having two mold sets one above another. Furthermore, the shorter profile allows easier accessability to the molds and carriers for maintenance and changeover. Horizontal design may also allow for increasing production requirements. If a continuous mold chain is utilized instead of shuttling the molds at a high rate of speed in nonforming sections, it is possible to mold corrugators in two directions. Accordingly, a single corrugator could have output coming from at least a first mold tunnel and a second mold tunnel. Another advantage of the horizontal design is the ability to quickly change mold blocks in the carriers which allows a single corrugator to manufacture a plurality of different products by swapping out the mold blocks on the carriers.
The mold blocks may be cooled in a number of ways. The traditional water cooling techniques may be utilized. Ambient air cooling, such as may be achieved by passing the open mold blocks through space when not molding, may also be utilized. If this cooling is not sufficient alone, forced air cooling may also be utilized. One method of forced air cooling includes the use of vortex nozzles. The vortex nozzles may be mounted in locations which allow for optimum thermal transfer of heat away from the mold. Utilizing cooling vortex nozzles, the traditional method of water cooling within the mold blocks may be eliminated.
A second method of cooling which may be utilized includes spraying the mold blocks with a solution (such as water or an evaporable solution). Using this second method, the thermodynamics of the heat of vaporization of the evaporating substance is utilized to increase the efficiency of the cooling process.
The corrugator of the preferred embodiment is also modularly constructed in sections. These sections interlock with one another. Accordingly, a single carrier design may be utilized regardless of a particular configuration length needed. A machine may be taken apart at a location, section(s) inserted, and the machine reconnected with the new section installed. In this manner, a single machine may be configured for different lengths or types of products. Modifications may be performed in a short period of time. Also, parts of like machines can be interchangeable. Furthermore, machines may be combined in tandem for ultra high-speed production and then separated for standard rate production. Modularity is also an advantage as components can be sold directly to the customer for in-house changes. Modularity allows for higher volume production of like parts which may reduce the individual part costs.
Additional objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description or accompanying drawings, or may be learned through the practice of the invention.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.